Apparatus for the excitation of electrodeless discharge tubes

ABSTRACT

High efficiency apparatus for exciting electrodeless discharge tubes in which the tube is placed partially inside a cylindrical resonant cavity with the internal metal coated area of the discharge tube in a gap between the cavity and a sleeve having its axis in longitudinal alignment with the axis of the cavity, and a capacitor plate outside the cavity is movable towards and away from the cavity axis in order to tune the cavity to the exact frequency of the energizing supply.

United States Patent Inventor Vivian R. Williamson Braintree, England Appl. No. 885,602

Filed Dec. 16, 1969 Patented Aug. 17, 1971 Assignee Evans Electroselenium Limited Bralntree, England Priority Dec. 23, 1968 Great Britain 61 155/68 APPARATUS FOR THE EXCITATION OF ELECTRODELESS DISCHARGE TUBES 1 1 Claims, 3 Drawing Figs.

U.S. Cl 333/83 R, 313/201, 313/234, 315/39, 333/13 Int. Cl 1101p 7/06, l-iOlj 11/00, 1-10lj19/80 Field of Search"; 333/83, 13,

References Cited UNITED STATES PATENTS Bitner Robinson Miller Lampert et al. Painter Grojean Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorney-Berman, Davidson and Berman 313/201 X 313/201 X 333/33X 333/81 333/83 356/85X ABSTRACT: High efficiency apparatus for exciting electrodeless discharge tubes in which the tube is placed partially inside a cylindrical resonant cavity with the internal metal coated area of the discharge tube in a gap between the cavity and a sleeve having its axis in longitudinal alignment with the axis of the cavity, and a capacitor plate outside the cavity is movable towards and away from the cavity axis in order to tune the cavity to the exact frequency of the energizing supply.

PATENTEDAUGIYIHYI 3,600,712

SHEET 1 UF 2 Inventor VII IAN R. W/LL/AMSON PAIENTED AUG] 7 IHYI SHEET 2 OF 2 Invenior. V/V/AN A, WILL mMso/v APPARATUS FOR THE EXCITATION OF ELECTRODELESS DISCHARGE TUBES This invention relates generally to analysis by atomic absorption spectroscopy and atomic fluorescence spectroscopy, and more particularly to apparatus for the excitation of electrodeless discharge tubes used in these methods of analysis.

In the process of analysis by atomic absorption spectroscopy and atomic fluorescence spectroscopy it is necessary to produce radiation at a particular frequency and this is often produced by the use of an electrodeless discharge tube, which is normally in the form of a sealed glass tube containing a spot of pure metal on the inner surface of the tube and a minute amount of a rare gas, such as neon or argon. This is subjected to a high frequency field to produce ionization of the residual gas in the tube and hence produce a discharge at the characteristic wavelength of the pure metal in the tube. This discharge is, in the course of the analysis, usually directed to a flame in which the material being analyzed is vaporized.

In order to excite the electrodeless discharge tube it is usual to subject the tube to the influence of a high frequency electrical field and the object of the invention is to provide apparatus for exciting such a tube which is so constructed that field of maximum intensity is produced from given apparatus so that the apparatus as a whole functions at its maximum efficiency.

The invention consists of an excitation apparatus for electrodeless discharge tubes comprising a resonant cavity, a sleeve coaxial with the cavity and spaced from the high potential end thereof, means to retain an electrodeless discharge tube in the sleeve with the internal metal coated area of the discharge tube in the gap between the cavity and the sleeve, and at least one capacitor plate connected to the low potential end of the cavity movable towards and away from-the cavity axis adjacent the high potential end of the cavity in order to tune the cavity to the exact frequency of the energizing supply.

The sleeve may be movable with respect to the cavityand the means to retain the discharge tube in the sleeve may be in the form ofa spring clip.

Conveniently an outer metal casing is provided inwhich the low potential end of the cavity is secured and the casing may contain supports for the sleeve and the movable capacitor plate. The casing may be provided with a tapped hole coaxial with the cavity and the sleeve may be externally screw threaded so that it may be moved by screwing into and out of the casing. A locknut may be provided to lock the sleeve in its adjusted position.

The cavity is conveniently in the form of a cylindrical metal tube and the capacitor plate may be mounted on a screwthreaded spindle set on an axis perpendicular to the axis of the tube so that the capacitor plate may be screwed towards and away from the tube. A flat plate may be attached at the high potential end of the tube to cooperate with the capacitor plate. There may be two or more capacitor plates, depending upon the range of frequency over which tuning adjustment is required and there may be means to lock the or each capacitor plate in its adjusted position. A preferred embodiment of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional elevation of an apparatus according to the invention;

FIG. 2 is an end elevation looking in the direction of the arrow ll OF FIG. 1; and

FIG. 3 is a schematic diagram showing the general layout of a system which includes the apparatus according to the inventron.

Referring to the drawings the excitation apparatus comprises a resonant cavity in the form of a cylindrical metal tube 11. The tube 11 is fixed into an end plate 12 which forms a part of an outer casing and is provided with a register 13. A tube 14 constituting a further part of the outer casing fits on to the register 13 and is fixed into a tubular member 15 (for example by soldering), the tubular member 15 being secured to the end plate 12 by screws l6.

The tube 14 has a further tubular member 17 attached at its other end (also by soldering) and another end plate 18 having a register 19 upon which the tube 14 is fitted is secured to the tubular member 17 by screws 20.

The second end plate 18 is internally screw-threaded at 21 and a sleeve 22 is externally screw-threaded so that the sleeve 22 is coaxial with the cavity 11. The gap 23 between the cavity 11 and the sleeve 22 may be varied by screwing the sleeve 22 into and out of the second end plate 18 and the sleeve may be locked in an adjusted position by means of a locknut 24. An end piece 25 is fixed to the outer end of the sleeve 22 and carries a spring clip 26 secured to the end piece 25 by screws 27.

An electrodeless discharge tube 47 may be pushed into the sleeve 22 so that it is retained by the spring clip 26 and adjusted so that the internal metal coated area of the tube lies in the gap 23.

A coaxial cable connector, generally indicated by reference 28, is fixed in the tubular member 15 and the insulated connection projects through the cavity 11 so that an electrode, indicated in dotted lines at 29, may be fitted.

A capacitor plate 30 in the form of a metal disc is fixed to a screwthreaded spindle 31 and is carried in an internally screw-threaded metal bush 32 attached to the outer casing tube 14. The spindle 32 is provided with an outer knob 33 so that it may easily be rotated by hand and the bush 32 is slotted at 34 and provided with a locking screw 35 so that the capacitor plate 30 may be locked in an adjusted position. A second assembly identical with the assembly consisting of elements 30 to 35 is generally indicated by reference 36.

Since the cavity 11 is in the form of a tube flat circular electrode plates 30a are conveniently fixed adjacent the inner end of the cavity 11 so that the effectiveness of the capacitors constituted by the plates 30 and 30a is the maximum.

A slot 37 is formed in the outer tube 14 through which radiation from the portion of the discharge tube lying in the gap 23 may pass out of the apparatus.

In FIG. 3 the apparatus according to the invention is indicated by reference 38. Energization of the apparatus is provided by a magnetron 39 connected to a power supply which is not shown and which is tuned to produce a high frequency supply at 2,450 Mc/s per second :25 Mc/s per second. This is conducted by a coaxial cable 40 to a directional coupler 41 and the directional coupler is connected by a further coaxial cable 42 to the apparatus 38. The branch from the directional coupler 41 is connected via another coaxial cable 43 to a detector 44 from which a lead 45 extends to a reflected power meter 46.

In operation, the apparatus according to the invention is energized by the magnetron 39. With the electrodeless discharge tube in position in the gap 23 the gap may be adjusted so that its length is such as to produce the most effective discharge from the discharge tube. The discharge from the tube may, in case of difficulty, be started by pushing an electrode (not shown) through the slot 37 to touch the discharge tube, the electrode being connected to a Tesla coil.

The cavity 11 is conveniently a foreshortened three-quarter wavelength cavity at the frequency mentioned above but in practice it is found that its resonant frequency may vary somewhat due to various features. One of these is the presence of the electrodeless discharge tube and it is also found that the loading imposed by the tube varies as it warms up and this pulls the frequency to some extent. This difficulty is overcome when using the apparatus according to the invention by adjusting the capacitor plate or plates 30 so that the cavity is exactly tuned to the output frequency of the magnetron 39. The effectiveness of the adjustment may be watched on the reflected power meter 46 since the most efficient operation will result in maximum power absorption within the apparatus 38 and minimum reflected power indicated on the reflected power meter 46. I

The two capacitor plates need not be set in positions at right angles to each other as shown in FIG. 2 and the coaxial cable connector 28 need not be at right angles to or in line with either of the capacitor plates.

1 claim:

excitation apparatus for electrodeless discharge tubes cavity so as to form a gap therebetween, means to retain the electrodeless discharge tube in the sleeve with the internal 7 metal coated area of the discharge tube in the gap between the cavity and the sleeve: means to apply a high frequency energizing supply to the cavity; and at least one capacitor plate outside the cavity and movable towards and away from the cavity axis to tune the cavity to the exact frequency of the energizing supply.

2. Apparatus as claimed in claim 1 in which the sleeve is movable with respect to the cavity.

3. Apparatus as claimed in claim 1 in which the means to retain the discharge tube in the sleeve is in the form of a spring clip.

4. Apparatus as claimed in claim 1 comprising an outer metal casing in which the low potential end of the cavity is secured.

5. Apparatus as claimed in claim 4 comprising supports for the sleeve and the movable capacitor plate within the casing.

6. Apparatus as claimed in claim 4 in which the casing is provided with a tapped hole coaxial with the cavity and the sleeve is externally screw-threaded to fit the tapped hole so that thesleeve may be moved by screwing into and out of the casing.

7. Apparatus as claimed in claim 6 comprising a locknut to lock the sleeve in its adjusted position.

8. Apparatus as claimed in claim 7 in which the cavity is in the form of a cylindrical metal tube and the capacitor plate is mounted on a screw-threaded spindle set on an axis perpen- I dicular to the axis of the tube so that the capacitor plate may be screwed towards and away from the tube. 4

9. Apparatus as claimed in claim 1 comprising a flatplate attached to the inner end of the cavity to cooperate with the capacitor plate.

10. Apparatus as claimed in claim 1 in which there are two or more capacitor plates in dependence upon the frequency range over which the tuning adjustment is required.

11. Apparatus as claimed in claim 1 comprising means to lock each capacitor plate in its adjusted position. 

1. An excitation apparatus for electrodeless discharge tubes of the kind containing a metal coating on a part of the tube interior surface comprising a cylindrical resonant cavity having a central axis; a cylindrical sleeve having its axis in longitudinal alignment with the axis of the cavity and spaced from the cavity so as to form a gap therebetween, means to retain the electrodeless discharge tube in the sleeve with the internal metal coated area of the discharge tube in the gap between the cavity and the sleeve: means to apply a high frequency energizing supply to the cavity; and at least one capacitor plate outside the cavity and movable towards and away from the cavity axis to tune the cavity to the exact frequency of the energizing supply.
 2. Apparatus as claimed in claim 1 in which the sleeve is movable with respect to the cavity.
 3. Apparatus as claimed in claim 1 in which the means to retain the discharge tube in the sleeve is in the form of a spring clip.
 4. Apparatus as claimed in claim 1 comprising an outer metal casing in which the low potential end of the cavity is secured.
 5. Apparatus as claimed in claim 4 comprising supports for the sleeve and the movable capacitor plate within the casing.
 6. Apparatus as claimed in claim 4 in which the casing is provided with a tapped hole coaxial with the cavity and the sleeve is externally screw-threaded to fit the tapped hole so that the sleeve may be moved by screwing into and out of the casing.
 7. Apparatus as claimed in claim 6 comprising a locknut to lock the sleeve in its adjusted position.
 8. Apparatus as claimed in claim 7 in which the cavity is in the form of a cylindrical metal tube and the capacitor plate is mounted on a screw-threaded spindle set on an axis perpendicular to the axis of the tube so that the capacitor plate may be screwed towards and away from the tube.
 9. Apparatus as claimed in claim 1 comprising a flat plate attached to the inner end of the cavity to cooperate with the capacitor plate.
 10. Apparatus as claimed in claim 1 in which there are two or more capacitor plates in dependence upon the frequency range over which the tuning adjustment is required.
 11. Apparatus as claimed in claim 1 comprising means to lock each capacitor plate in its adjusted position. 